1. Field of Invention
The invention relates to a method for operating a wind energy installation having a tower, a nacelle which is arranged on the tower and the azimuth of which can be adjusted, and a rotor having at least one rotor blade, the blade angle of which can be adjusted, tower oscillations being detected and monitored during operation of the wind energy installation using at least one measuring apparatus and the power operation of the wind energy installation being switched off if a measurement variable, in particular a sliding average, of the tower oscillations exceeds a tower oscillation limit value. The invention also relates to a wind energy installation.
2. Brief Description of Related Art
Corresponding wind energy installations having a tower, on the top of which is arranged a nacelle with a rotor, are structures which are very prone to oscillation. Oscillations may be excited by asymmetries in the installation, for example incorrect blade positions, aerodynamic or mechanical imbalances, or by the tower dam effect, but also by adverse environmental conditions, for example extraordinary turbulence outside the design specifications, by ice build-up or by variable inflow conditions. These tower oscillations stress the material of the tower during operation of a wind energy installation and are taken into account when designing the load-bearing capacity of the tower for a planned service life of the tower.
In order to prevent the occurrence of tower oscillations which stress the tower beyond the acceptable degree, tower oscillations are usually monitored and the load operation of the installation is switched off if a permanently predefined limit value which is specific to the installation is exceeded. This prevents excessive oscillation events from shortening the service life of the tower beyond a tolerable degree.
Two permanently parameterized switch-off thresholds for the tower acceleration have previously usually been implemented, one with a short switch-off time, for example one to two half-oscillations of the tower and with a higher limit value, and one with a somewhat longer switch-off time, for instance five to six half-oscillations and with a low limit value. Since conventional tower natural frequencies are 0.2 to 0.5 Hz, the observed duration is therefore 2.5 to 5 seconds for the short period and 7.5 to 15 seconds for the longer period. In this case, the limit values must be so highly parameterized that no triggering is carried out even in the case of a switch-off wind in the normal situation, that is to say at high wind speeds at which the wind energy installation is regularly switched off. It is also known in the prior art to reduce the power and/or rotational speed of a wind energy installation when high oscillation levels occur in order to reduce the oscillation level without switching off the wind energy installation.